A. Field of the Invention
The invention relates to the field of antibodies. Specifically, the invention relates to a bispecific antibody comprising two antibody variable domains on a single polypeptide chain. The invention further relates to the use of such a bispecific antibody for the preparation of a pharmaceutical composition. The invention further relates to a method for the prevention, treatment or amelioration of a disease comprising administration of an effective amount of such a bispecific antibody. Finally, the invention relates to a kit comprising such a bispecific antibody.
B. Related Art
Unifying two antigen binding sites of different specificity into a single construct, bispecific antibodies have the ability to bring together two discreet antigens with exquisite specificity and therefore have great potential as therapeutic agents. This potential was recognized early on, leading to a number of approaches for obtaining such bispecific antibodies. Bispecific antibodies were originally made by fusing two hybridomas, each capable of producing a different immunoglobulin. The resulting hybrid-hybridoma, or quadroma, was capable of producing antibodies bearing the antigen specificity of the first parent hybridoma as well as that of the second parent hybridoma (Milstein et al. (1983), Nature 305:537). However, the antibodies resulting from quadromas often exhibited undesired properties due to the presence of an Fc antibody portion.
Largely due to such difficulties, attempts later focused on creating antibody constructs resulting from joining two scFv antibody fragments while omitting the Fc portion present in full immunoglobulins. Each scFv unit in such constructs was made up of one variable domain from each of the heavy (VH) and light (VL) antibody chains, joined with one another via a synthetic polypeptide linker, the latter often being genetically engineered so as to be minimally immunogenic while remaining maximally resistant to proteolysis. Respective scFv units were joined by a number of techniques including incorporation of a short (usually less than 10 amino acids) polypeptide spacer bridging the two scFv units, thereby creating a bispecific single chain antibody. The resulting bispecific single chain antibody is therefore a species containing two VH/VL pairs of different specificity on a single polypeptide chain, wherein the VH and VL domains in a respective scFv unit are separated by a polypeptide linker long enough to allow intramolecular association between these two domains, and wherein the thusly formed scFv units are contiguously tethered to one another through a polypeptide spacer kept short enough to prevent unwanted association between, for example, the VH domain of one scFv unit and the VL of the other scFv unit.
Bispecific single chain antibodies of the general form described above have the advantage that the nucleotide sequence encoding the four V-domains, two linkers and one spacer can be incorporated into a suitable host expression organism under the control of a single promoter. This increases the flexibility with which these constructs can be designed as well as the degree of experimenter control during their production.
Remarkable experimental results have been obtained using such bispecific single chain antibodies designed for the treatment of malignancies (Mack, J. Immunol. (1997), 158:3965–70; Mack, PNAS (1995), 92:7021–5; Kufer, Cancer Immunol. Immunother. (1997), 45:193–7; Löffler, Blood (2000), 95:2098–103) and non-malignant diseases (Brühl, J. Immunol. (2001), 166:2420–6). In such bispecific single chain antibodies, one scFv unit is capable of activating cytotoxic cells, for example cytotoxic T cells, within the immune system by specifically binding to an antigen on the cytotoxic cells, while the other scFv unit specifically binds an antigen on a malignant cell intended for destruction. In this way, such bispecific single chain antibodies have been shown to activate and redirect the immune system's cytotoxic potential to the destruction of pathological, especially malignant cells. In the absence of such a bispecific single chain antibody construct, malignant cells would otherwise proliferate uninhibited.
However, bispecific single chain antibodies must fulfil additional requirements. In order to achieve the desired activity, each scFv unit of a bispecific single chain antibody should remain properly folded, something which often proves unrealisable in conventional bacterial expression systems such as E. coli. The need to use less conventional, more cumbersome and more costly eukaryotic—even mammalian—expression systems often complicates the production of bispecific single chain antibodies and/or reduces the amount of product obtainable to levels lower than desired for therapeutic application.
In the event that a bispecific antibody is intended for therapeutic use, it is desirable to produce high amounts of this antibody solubly and in the desired functional form. The production of functionally active antibody becomes especially critical when producing bispecific antibodies of which one portion is able to activate and recruit the cytotoxic potential of human immune effector cells. For example, a produced antibody devoid of functional activity will not lead to the desired activation of human immune effector cells, while a bispecific antibody which is functionally active, albeit not in the desired manner, as for example may be the case when the bispecific antibody is produced in a heterogeneous form containing multiple isomers, may activate and recruit the cytotoxic potential of human immune effector cells in unforeseeable and/or unintended manners.
One example of the sort of unintended activation mentioned above is the possibility of activation of human immune effector cells to exert an effect on other human immune effector cells instead of on a target cell intended for destruction. This type of immune effector cell fratricide may jeopardize the effectiveness of a regimen of therapy depending on the activity of human immune effector cells.
However, reliable production of large amounts of functional single chain antibody, especially large amounts of functional bispecific single chain antibody, from prokaryotic expression systems such as E. coli is often limited, necessitating costly optimization (Baneyx (1999), Curr. Opinions Biotechnol. 10:411–21).
In summary, bispecific antibody constructs can be of great therapeutic use in redirecting the powerful potential of the body's own immune system to achieve the destruction of diseased cells. By the same token, however, the activation of such a powerful means of eradicating or neutralizing unwanted cells requires that this power be controlled as precisely as possible so that the cytotoxic potential of the immune system is recruited and applied only in the direction intended and no other.
Clearly, when one specific binding arm of a bispecific single chain antibody is to recruit the activity of a human immune effector cell, for example a cytotoxic T cell, there exists an especially heightened and, as yet, unmet need for bispecific single chain antibodies which overcome limitations as described above.